1. Field of the Invention
The present invention relates to a variable-voltage and variable-frequency electric power converter for a load such as a motor, and a method of controlling the same.
2. Description of the Related Art
A conventional power converter comprises a DC smoothing capacitor serving as a DC voltage source, a first inverter, a second inverter, an output transformer, a load device, a load current detector, a comparator, a current control/compensation circuit, a proportional amplifier, pulse-width modulation (PWM) controllers, etc.
The first and second inverters are PWM control inverters for converting a DC voltage into a variable-voltage and variable-frequency AC voltage. The first inverter generates output voltage V.sub.L1 through the output transformer. The second inverter directly generates output voltage V.sub.L2. The load device is applied with a voltage of V.sub.L =V.sub.L1 +V.sub.L2. Voltage V.sub.L is adjusted to control load current I.sub.L.
When output frequency f.sub.0 is zero, since no voltage V.sub.L1 can be generated from the output transformer, the second inverter controls load current I.sub.L, and when frequency f.sub.0 is increased to a certain level, the first inverter controls load current I.sub.L in turn.
FIGS. 17 and 18 show load terminal voltage V.sub.L and input signals V.sub.1 * and V.sub.2 * of the PWM controllers of the first and second inverters as a function of output frequency f.sub.0 in the conventional power converter.
When load current I.sub.L is assumed to be constant, load terminal voltage V.sub.L is expressed by output frequency f.sub.0 as follows: EQU V.sub.L =V.sub.C +I.sub.L .multidot.R.sub.L +j.omega..sub.0 L.sub.L I.sub.L ( 1)
V.sub.C : counter electromotive force of load (for motor) PA1 R.sub.L : resistance of load PA1 L.sub.L : inductance of load PA1 .omega..sub.0 : 2.pi.f.sub.0
A broken line in FIG. 17 indicates a voltage drop caused by resistance R.sub.L of the load.
A broken line in FIG. 18 indicates input signal V.sub.1 * of the PWM controller of the first inverter. V.sub.1 *=0 is established for f.sub.0 .ltoreq.f.sub.min, and V.sub.1 * is proportional to f.sub.0 in a region of f.sub.0 &gt;f.sub.min. A solid line in FIG. 18 indicates input signal V.sub.2 * of the PWM controller of the second inverter. Signal V.sub.2 * is controlled to output voltage V.sub.L given by equation (1) for f.sub.0 &lt;f.sub.min, and V.sub.2 *=I.sub.L *.multidot.R.sub.L =constant in a region of f.sub.0 &gt;f.sub.min.
In this manner, in the conventional power converter, the second inverter need only generate required voltage V.sub.L up to minimum frequency f.sub.min at which the output transformer can be operated, and can have a smaller capacity than the first inverter. Since the first inverter generates voltage V.sub.L1 through output transformer, serial or parallel multiplexed operations of this inverter can be easily performed, and its capacity can be easily increased. Therefore, the converter is convenient for one for driving a large-capacity AC motor.
According to the above conventional power converter, when output frequency f.sub.0 becomes high and load current I.sub.L is to be controlled by the first inverter, the following problems are posed.
In general, secondary voltage V.sub.L1 of the output transformer is proportional to output voltage V.sub.1 of the first inverter. However, when a DC component is included in load current I.sub.L due to, e.g., drift of the current detector the output transformer gradually suffers from DC polarized magnetization, and soon, its iron core is magnetically saturated to one side. As a result, secondary voltage V.sub.L1 of the output transformer is decreased, and load current I.sub.L is decreased to be lower than instruction value I.sub.L *. Then, the inverter operates to increase voltage V.sub.1 and further saturate the iron core. Therefore, primary current I.sub.1 of the output transformer is abruptly increased, and may become an overcurrent to destroy elements of the first inverter. In order to prevent polarized magnetization of the output transformer, the sectional area of the iron core may be increased and an air gap is provided. In this case, the weight and dimensions of the output transformer are increased, resulting in an uneconomical system.